The present invention concerns a method in accordance with the preamble of claim 1 for treatment of cellulose pulps, in particular pulps prepared by the kraft process and by extended pulping processes.
The invention also concerns an enzymatic preparation according to the preamble of claim 17 as well as a method for isolating enzyme-producing strains and a method for producing a desired enzyme.
In traditional chlorine bleaching the residual lignin of cellulose pulps is solubilized by using chlorine or chlorine dioxide. Presently the pulps are frequently also bleached by oxygen gas, hydrogen peroxide, ozone or by combined sequences including these substances as well as the above-mentioned traditional bleaching chemicals. Enzymatic treatments, carried out by hemicellulases or lignin degrading enzymes, have been combined with the traditional and new bleaching sequences leading to increased bleachability of the fibres. The amounts of enzymes needed to achieve the improved bleachability are low, and the enzymatic treatment can easily be incorporated into the pulp production processes.
Conventionally, the enzymatic treatments have been performed directly on fibres from the pulping processes. In an earlier PCT patent application, published under number WO 93/11296, we have shown that the properties of the cellulosic fibres profoundly affect the possibilities of enzymes to act efficiently on the pulps. Thus, for instance, the action of the xylanases is not optimal when the surface charge, i.e. the zeta potential, is very low. Furthermore, a kraft pulp treated at a low pH-value, at which the carboxylic groups of the hemicelluloses present in the pulp are in an acid form (and do not contain metal counter ions), constitutes a rather poor substrate for enzymatic (hemicellulase) treatments. By enzymatically removing the carboxylic groups of the hemicelluloses from the cellulose pulps, both the surface charge and the metal-ion content of the pulp can be changed. According to a method of our earlier patent application, the methyl glucuronic acid groups, can be removed by treating the cellulose pulp with an enzyme preparation having an essential glucuronidase enzyme activity. That activity will cleave the bond between the xylose unit of the xylan chain and the carboxylic acid side group, whereby the carboxylic group will be removed.
Although quite promising results have been obtained with the glucuronidase enzyme treatment, there is still a need for further improving the modification of pulps, in particular kraft pulps and some modified (extended) cooking pulps, containing very small amounts of glucuronic acids.
In connection with the present invention it has surprisingly been found that the acidic side chains of pulp xylan are not exclusively composed of 4-O-methyl-xcex1-D-glucuronic acid or xcex1-D-glucuronic acid, as presently believed, but during pulping by the conventional kraft method as well as by some of the new, extended cooking methods an essential part of the 4-methyl glucuronic acid (in the following shortened MeGluA) is in fact converted to an unsaturated derivate thereof, viz. 4-deoxy-xcex1-L-threo-4hexenuronic acid, or hexenuronic acid, (HexA). This carboxylic acid group can, depending on the aching conditions, also be found in the bleached pulp.
With reference to the above findings, the present invention provides a novel solution for modification of industrial pulps, which is based on the concept of selectively removing at least a part of the hexenuronic acid groups contained in the pulp.
In particular, the method according to the present invention is characterized by what is stated in the characterizing part of claim 1.
The enzyme preparation according to the present invention is characterized by what is stated in the characterizing part of claim 17. The method for isolating microorganism strains producing hexenuronidase activity is characterized by what is stated in the characterizing part of claim 22 and the method for producing hexenuronidase by what is stated in the characterizing part of claim 24.
For the purpose of the present invention the term xe2x80x9cglucuronic acid groupsxe2x80x9d is used as an abbreviation of 4-O-methyl-xcex1-D-glucuronic acid or xcex1-D-glucuronic acid groups. xe2x80x9cHexenuronic acid groupsxe2x80x9d is an abbreviation of 4deoxy-xcex1-L-threo-4-hexenuronic acid groups.
The term xe2x80x9cenzyme preparationxe2x80x9d denotes any product which contains at least one enzyme. Thus, such an enzyme preparation may be a culture liquor containing one or more enzymes, an isolated enzyme or a mixture of one or more enzymes. In addition to the enzymatic activity such a preparation preferably contains adjuvants which commonly are used in enzyme preparations intended for application in the paper and pulp industry. Such adjuvants are typically comprised of, for instance, buffering agents and stabilizing agents.
The term xe2x80x9chexenuronidasexe2x80x9d as used herein, refers to an enzyme which is capable of removing hexenuronic acid groups which are attached to xylose units. Similarly, xe2x80x9cglucuronidasexe2x80x9d is an enzyme capable of removing glucuronic acid groups attached to xylose units. The action of the hexenuronidase can be based on hydrolytic action, which cleaves the bond between the acid groups and the xylose units. Alternatively, the hexenuronidase can act on the unsaturated HexA ring, in particular the double bond thereof, destabilizing or breaking up the structure.
The term xe2x80x9curonidaesxe2x80x9d covers both hexenuronidase and glucuronidase.
xe2x80x9cA substantial amount of hexenuronidasexe2x80x9d or xe2x80x9csubstantial amount of glucuronidasexe2x80x9d indicates that the hexenuronidase and glucuronidase activities of the enzyme preparation are comparatively high, i.e. that the amounts are sufficient to release the uronic acid groups from the xylose units. In particular, the uronidase activities of the enzyme preparation should be so high that a substantial part of the uronic acid groups of the substrate can be removed by contacting the substrate with the enzyme.
xe2x80x9cKraft pulpingxe2x80x9d is used synonymously with xe2x80x9csulphate cookingxe2x80x9d and it designates the cooking method in which sodium sulphide and sodium hydroxide are used as principal cooking chemicals. xe2x80x9cNewxe2x80x9d or xe2x80x9cmodifiedxe2x80x9d cooking methods are represented by pulping methods which are based on continuing the conventional kraft cooking until the Kappa number of the lignocellulosic material is reduced to below about 20. These methods typically include an oxygen treatment.